1. Field of the Invention
The present invention relates to test and measurement systems. More specifically, the present invention relates to systems employed to make amplitude and phase measurements of antennas in near field range, far field range, and compact range measurement systems.
While the present invention is described herein with refernce to a preferred embodiment in an illustrative application, it is to be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings of the present invention will recognize additional modifications, embodiments and applications within the scope thereof.
2. Description of the Related Art
The high cost associated with the manufacture and launch of satellite systems and the subsequent inaccessibility thereof makes it imperative that the satellite be designed in all respects to provide reliable performance throughout its useful life. As a key component of the system, the antenna and its design must be proven to be satisfactory prior to launch. Accordingly, antenna test and measurement systems are used to fully exercise the antennas in their many modes of operation.
Test ranges are currently classified as one of three types: (1) far field ranges; (2) near field ranges; and (3) compact ranges.
The far field range is an open loop system in which the test equipment is physically mounted a sufficient distance from the antenna under test to detect and measure its actual beam pattern. In far field ranges, it is typically sufficient to measure the amplitude or gain of the antenna.
In near field ranges, the test equipment is mounted in close physical proximity to the antenna under test. In near field ranges, it is typically necessary to make phase as well as amplitude measurements from which the actual antenna beam pattern may be extrapolated. The near field systems are typically closed loop in that the activation and positioning of the transmit antenna and the collection of data from the antenna under test are under the control of a single system controller.
In a compact test range, a reflector is used between the antenna under test and the measurement apparatus to simulate the longer path length of a far field range.
In each system the tests performed are typically quite extensive. Input and/or output beam characteristics are often tested as a function of many variables, parameters ad operating conditions. The amplitude or gain, for example, may be measured as a function of frequency, azimuth, elevation, distance and/or time. (Amplitude measurements can be translated to gain measurements given the typically available reference input signal amplitude.) Numerous combinations and permutations of variables may be used as part of a single testing program.
In addition, where the antenna is of the phased array variety having a plurality of sensing and/or receiving elements (feeds), beam forming networks, input andoutput channels and/or input and output ports, the test may involve switching various combinations of feeds through various combinations of channels and ports. It is often desirable to run such tests using any of several signal polarization states and frequencies.
It is not difficult then to imagine how in some tests, as many as ten million gain measurements alone may be required. This is typically a time consuming process requiring specialized equipment and personnel skilled in the field. As such, the testing of the antenna often adds significantly to the development and manufacturing cost of the overall system. There is therefore a generally recognized need in the art to minimize the time required to completely test such antennas.
There is a countervailing need to perform more and more tests as antennas become more capable and complex. For example, while the antenna scans in azimuth, it was typically necessary to make a gain measurement at certain ports, channels and feeds at a specific frequency. However, there has now been recognized a need to make such measurements while simultaneously hopping through many frequencies.
Conventional measurement systems have had difficulty meeting the demanding requirements of this application. The operating speed of conventional systems has been a primary limitation. Limitations on the speed of conventional systems may result from the scheme used to detect and measure input power or gain and/or the capability of the system to switch from one set of ports to another.
Many prior art systems also require a computer to control the complex system through the testing program. In such systems, the computer is required to set up and energize the antennas, switch channels in and out of the testing apparatus, make the desired measure ments, and process and store the resulting data. In order to provide for additional increases in system speed and operational capability, it is generally desirable to free the computer from such tasks.
Network analyzers have heretofore enjoyed limited utility in near field ranges. A network analyzer is a system that provides one input signal corresponding to the amplitude of an input signal usually relative to a reference signal, and a second output signal corresponding to the phase of the input signal relative to a reference signal. Network analyzers typically have a broad band input frequency range which substantially prohibits their use in far field ranges. That is, the wide input frequency range permits too many extraneous signals to reach the detectors to provide sufficiently accurate readings. Thus, use of a network analyzer for the near field forced the use of another system for the typically necessary far field range measurement. This significantly increases the cost of making both measurements.
Even in near field applications, network analyzers typically required elaborate control software and switching systems to which it was difficult to add additional channels.
A need was therefore recognized in the art to provide a flexible system that could easily be used in both near and far field test ranges. Such a system is presently provided by the Scientific Atlanta model 2020. This system provides amplitude and phase measurements in the near field range and amplitude measurements in the far field range.
However, such systems typically suffer several shortcomings. For example, a phase locked loop is typically employed to lock on to and measure the amplitude and phase of the input signal. As such, these systems are typically too slow for frequency hopping applications.
Secondly, the Scientific Atlanta systems typically require substantial computer interaction. That is, the computer is required to input each data point individually, as well as perform all other functions one at a time. This wastes considerable time in transmitting commands and data back and forth along the bus in addition to waiting for replies where necessary. This is undesirable for the reasons mentioned above, namely, tasking the computer forces a limitation on the overall speed of the system.
A third shortcoming of such systems is due to the fact that the system fo port switching only allows for switching between two ports at a time. To use such systems in applications requiring additional ports and/or channels requires the purchase of additional switching systems.
Thus, relative to the related art, there is a general need for an antenna test system that is useful in both the near, far, and compact ranges which is fast, flexible, and requires minimal interaction with the system control computer.